The present invention relates generally to methods for performing cementing operations in a well bore and cementing compositions for use in such methods. More particularly, the present invention relates to methods and compositions that are suitable for operations requiring a lightweight cement and that are performed in high temperature well conditions.
High temperature subterranean wells, for example, geothermal wells, often contain carbon dioxide. Because of the high static well bore temperatures involved, coupled with the presence of fresh or brine waters containing carbon dioxide, or hydrocarbons containing carbon dioxide, conventional hydraulic well cements rapidly deteriorate due to alkali carbonation, especially sodium carbonate induced carbonation. Further, severe corrosion of steel pipe takes place thereby resulting in the total disruption of the conventional cement supported well structure. In geothermal wells, which typically involve very high temperatures, pressures and carbon dioxide concentrations, conventional well cement failures have occurred in less than five years causing the collapse of the well casing.
It has heretofore been discovered that a cement material known as calcium phosphate cement formed by an acid-base reaction between calcium aluminate and a phosphate-containing solution has high strength, low permeability and excellent carbon dioxide resistance when cured in hydrothermal environments. However, calcium phosphate cement has a relatively high density, e.g., a density in the range of from about 14 to about 17 pounds per gallon, which is too high for some geothermal applications. That is, in geothermal wells, the hydrostatic pressure exerted by normal density calcium phosphate cement can exceed the fracture gradients of subterranean zones penetrated by the well bore. Consequently, fractures are created in the formation and cement enters into such fractures and is lost.
Thus, there is a need for lightweight well cementing compositions useful in performing cementing operations under high temperature conditions.